Integrated circuits are often manufactured in and on silicon and other semiconductor wafers. Such integrated circuits include literally millions of metal oxide semiconductor (MOS) field effect transistors. Such MOS transistors may include p-channel MOS (PMOS) transistors, and n-channel MOS (NMOS) transistors, depending on their dopant conductivity types.
Wafers may be obtained by drawing an ingot of silicon out of a liquid silicon bath. The ingot may be made of monocrystalline (single-crystal) silicon, and is subsequently sawed into individual wafers. A layer of silicon may be then deposited over each wafer. Because the wafer may be made of monocrystalline silicon, the deposition conditions can be controlled so that the layer of silicon deposits “epitaxially” over the wafer. “Epitaxy” refers to the manner in which the silicon layer deposits on the wafer—the layer of silicon has a lattice which has a structure which follows a structure of a lattice of the monocrystalline silicon of the wafer. The layer of silicon may be also substantially the same material as the monocrystalline silicon of the wafer, so that the lattice of the silicon layer also has substantially the same spacing as the spacing of the lattice of the monocrystalline silicon of the wafer.
A gate dielectric layer, a gate electrode, and spacers are subsequently formed on the layer of silicon. Ions are also implanted into the layer of silicon, which form source and drain regions on opposing sides of the gate electrode. A voltage can be applied over the source and drain regions. Current flows from the source region to the drain region through a channel below the gate dielectric layer when a voltage is applied to the gate electrode.
It has been found that applying a strain to the channel may improve functionality of the transistor. One way to apply such a strain is to deposit a material with a different lattice spacing than monocrystalline silicon on the wafer. Depending on the lattice spacing, a tensile or compressive strain may be induced in the channel.